Vapor pressures of aqueous sodium dodecyl sulfate solutions have been measured at (298.15, 303.15, 308.15, and 313.15) K and at concentrations m(1) up to 0.03 mol of solute/kg of solution. The vapor pressure of the aqueous solution decreases with mi in the range of m(1) from (0 to about 0.005) mol . kg(-1) and exhibits negative deviation from Raoult’s Law. The vapor pressure increases with m(1) when 0.005 mol . kg(-1) < m(1) < 0.009 mol . kg(-1). The vapor pressure is approximately independent of m(1) for m(1) > 0.009 mol . kg(-1) (the critical micelle concentration). From the anomalous behavior of the vapor pressure, it is deduced that when mi is less than about 0.005 mol . kg(-1), the surfactants in the solution exist mainly in the form of monomers. When m(1) is in the range from (0.005 to 0.009) mol . kg(-3), the surfactant in the solution begins to aggregate even though the concentration is lower than the critical micelle concentration.